P0138 on a Mazda: Causes, Fixes & Repair Cost

🟢 Low severity · drivable

Safe to drive — this downstream sensor only monitors the catalytic converter, so a Mazda with P0138 can be driven while you arrange the repair.

What P0138 means on a Mazda

On a Mazda, P0138 means the engine computer sees high voltage from the bank 1 downstream oxygen sensor, the one after the catalytic converter. This sensor only checks converter efficiency, so high voltage is almost always a faulty sensor or a wiring short rather than a rich-running engine.

Is P0138 common on Mazdas?

P0138 is most common on higher-mileage Mazdas as the downstream sensor fails. Mazda3, Mazda6, and CX-5 models see it regularly, and wiring in the exhaust area can short as its insulation ages.

Commonly reported on: Mazda3, Mazda6, CX-5, CX-9.

Symptoms

Check engine light on
Usually no change in how it drives
Possible reduced fuel economy
Sometimes no noticeable driving symptoms

Common causes

most common: Faulty bank 1 downstream oxygen sensor
common: Sensor wiring shorted to power
common: Exhaust leak before the sensor
less common: Failing catalytic converter

How to fix it

Fix	DIY?	Typical cost
Replace the bank 1 downstream oxygen sensor	Yes	$60–230
Repair shorted sensor wiring	Mechanic	$70–250
Replace the catalytic converter if it's failing	Mechanic	$800–2500